The need to vertically hold a post or pole against the force of gravity occurs in many situations. Two principle examples known to the prior art where some degree of precision is required include the use of mounting optical elements on optical tables and camera tripods. The discussion of the prior art and the structure and operation of the invention will focus principally on holders for vertically oriented posts in the optical field, though it applies to devices such as camera tripods.
Posts and post holder arrangements have been used in the prior art for precisely positioning optical components on an optical test table. Optical components, such as lenses, mirrors and lasers, must be precisely oriented with respect to each other both in height and angular orientation. Typically, the optical components are held in a holder and mounted on a post and post holder arrangement.
FIG. 1 shows a post and post holder as it would appear on an optical table or breadboard. The optical table 10 is a rigid table which bears an array of threaded holes 12 on its surface. An optical component, such as a lens 14 is held in a holder 16 which is in turn mounted on a post 20 having a threaded end (not shown) held in a post holder 22. The post holder 22 is attached to the table 10 via the threaded holes 12. Generally, such a post and post holder arrangement provide two degrees of freedom, vertical and rotational about the axis of the post holder.
Referring to FIG. 2, the post 20 is a cylindrical rod made of suitable hard and rigid material, such as stainless steel. The width of the rod for most optical applications is on the order of one-half inch. Any length of rod desired may be used, and typically various lengths of post 20 are available. The post holder 22 has a central bore whose inside diameter is slightly larger than the outside diameter of the post 20. Double bore post holders exist in which two slightly non-concentric bores are made in the post holder 22 to receive the post 20. The double bore arrangement provides a vertical channel on the inside of the post holder 22 across from the screw 24 to provide precise positioning of the post 20 relative to the post holder 22. The post 20 is held relative to the post holder 22 by the screw 24. A knob 26 permits manual tightening of the screw 24, and optionally, a hole for an Allen wrench is provided in the face of knob 26 for tightening with an Allen wrench. Typically the screw is one quarter inch in diameter and has 20 threads per inch. Accordingly, the contact area between the tip of the screw 24 and the post 20 is in a relatively small area. Scoring of post 20 is possible if the post is not hard enough or if excessive force, such as through the use of an Allen wrench, is used to tighten the screw.
In operation, the user loosens the screw 24 via the knob 26 to permit free movement of post 20 in the post holder 22. Typically, the screw 24 should be backed out sufficiently to prevent scoring or scratching of the surface of the post 20 by the end of the end of the screw 24. The post 20 is positioned both vertically and at the proper angular orientation relative to the other components on the table 10. Once in position, the screw 24 is tightened locking the post 20 into place. If a double bore arrangement has been provided, the screw 24 forces the post 20 into contact along two line contacts running vertically in the post holder 22. In this way, more precise repositioning in the plane of the table 10 may be achieved.
While the post and post holder arrangement of the prior art has been in extensive and wide spread use, it suffers from a number of serious drawbacks. Most particularly, when the screw 24 is loosened to permit adjustment of the post 20, the gravitational force on the post 20 will cause it to drop if not supported. Typically, repositioning of the post 20 requires the use of two hands, one to loosen the knob and the other to position the post 20. Further, manual positioning of the post does not always insure positional accuracy. Slight movements of the posts may occur prior to locking of the post. Various attempts have been made to overcome these problems. For example, if the only desired degree of freedom is rotational freedom of the post 20, a collar may be placed around the post 20 which sits on top of the post holder 22. In this way, when the screw 24 is removed no vertical motion takes place. This arrangement however does not permit vertical movement of the post.
Telescoping post mounts using a rack and pinion arrangement permit vertical positioning, but no degree of freedom for angular orientation.
Tripods are often made with chuck arrangements where a clamping force is applied to the post. These too suffer from the basic problem that when the external clamping force is removed sufficient that the post can move vertically, the action of gravity causes the post to fall unless restrained.